Dual frequency antenna employing parabolic reflector



May 20, 1969 R. J. STEGEN 3,445,850

DUAL FREQUENCY ANTENNA EMPLOYING PARABOLIC REFLECTOR Filed Nov. 8, 1965 HORN FEED INVENTOR 9055i? J STEGE/V YSf/JW/ ATTORNEY United States Patent 3,445,850 DUAL FREQUENCY ANTENNA EMPLOYING PARABOLIC REFLECTOR Robert J. Stegen, Van Nuys, Calif., assignor to Canoga Electronics Corporation, Chatsworth, Calif., a corporation of Nevada Filed Nov. 8, 1965, Ser. No. 506,781 Int. Cl. H01q 21/00 US. Cl. 343727 3 Claims ABSTRACT OF THE DISCLOSURE The invention relates generally to broadband antennas for electromagnetic waves, and more particularly relates to an antenna which includes a paraboloidal reflector characterized by transmission and reception of broadband radiation, or in which the reflector has otherwise a single or double curvature surface. Generally these types of antennas require a feeding element at the focal point or effective focal point of the beam collimating reflector.

In the present invention, there is provided a reflector in which at least one cross-section thereof defines a parabola and there is a microwave feed horn positioned at or near the focal point of the reflector to provide transmission of a beam upon the surface of the reflector, and in which there is a series of dipole arrays which may be 2- dimensional crossed dipoles mounted upon a surface of the reflector along an imaginary intersecting plane thereof, and in which the arrays at their maximum direction of radiation generally are perpendicular to the surface of the reflector which is proximate thereto, and in which there is compensating means for phase error in the transmission lines which feed the series of dipole arrays so that the phase distribution across the aperture of the reflector is substantially constant.

An object therefore of the present invention is to provide a reflector antenna that operates over a broad frequency band or at multiple frequency bands which have superior performance to existing types and which employs a multi-frequency feed element or microwave feed means.

Another object of the present invention is to reduce the blocking aperture of the feed horn means so that the side lobe level that may be achieved thereby is improved.

A further object of the invention is to reduce the mechanical complexity involved in providing a conical scanner operating at generally VHF frequencies.

A further object of the invention is to reduce the blocking aperture so that the overall gain of the antenna system is substantially increased.

Another object of the invention is to employ those advantages of a two-dimensional array for some of the frequency bands, and in which these advantages are derived from the accurate control available over the aperture amplitude distribution. The advantages present themselves in the form of increased gain for the aperture under consideration and improved side lobe level.

As is generally well known in the prior art, an antenna system which consists of a paraboloidal reflector and feed is used to form a beam of radiation energy which has a desired directivity and side lobe level in the direction of the axis of the reflector.

3,445,850 Patented May 20, 1969 ice It is the concept of the present invention that the array of dipoles disposed on the surface of the reflector do not feed the reflector, but they are used as an effective reflecting plane to produce a beam of broadband, and in which the directivity pattern may be either a flat or narrow lobe so that the aperture due to the feed element or feed means is improved and controlled as may be desired.

A further object and advantage of the invention is to combine an array type dipole system of antennas with a parabolic reflector type antenna so that the elements of the array are mounted on the parabolic reflector surface which then serves as their ground screen.

Antennas which are to operate in the VHF and above frequency regions generally require a feed aperture which is an appreciable fraction of the diameter of the reflector. A feed means operating as a conical scanner or a monopulse type feed means would block such an appreciable fraction of the paraboloid area that it could cause severe changes in the output pattern and in the maximum gain and side lobe level.

If the feed means is to be a conical scanner, the mass of material which is necessary to be rotated at the focal point or effective focal point of the reflector, presents quite a formidable mechanical problem, as well as adverse effects on the electrical beam pattern. These problems have been partially overcome in the prior art quite at the expense of massive feed point structures as well as relatively low scan rates.

The present invention contemplates an antenna of broad-beam characteristics which it required that will operate in several discreet frequency bands as desired. These frequency bands may be overlapping or quite widely separated.

The invention contemplates that the foregoing objects and advantages as well as further objects and advantages will become apparent upon full consideration of the following detailed description and accompanying drawings in which:

FIG. 1 is a schematic and diagrammatic representation of the broadband antenna system of the present invention; and in the drawing there is shown a cross section of a surface of a paraboloidal reflector 10 having a microwave feed means 12, such as a feed horn, as used in a conical scanner or a monopulse antenna, or the like, so positioned at the focus of the surface of the reflector or generally at the effective focal point of the beam collimating reflector to provide a beam directed upon the paraboloidal reflector surface. The antenna may operate in the VHF and microwave frequency bands and it may be a fixed beam, a conical scanner, or a monopulse antenna. The antenna may have an aperture which is relatively large in terms of wave lengths such as 20 feet or greater in diameter. The conventional conical scanner or monopulse type of feed may be employed at the microwave frequencies and positioned at the focus of the reflector surface to provide the fixed, conical scanning or monopulse beam, which positioning is illustrated in the figure. The VHF or UHF antenna requirements are considered to be satisfied by mounting an array or series of twodimensional crossed dipoles 14 directly upon the surface of the paraboloidal reflector 10, as shown. The higher frequency antenna would be still mounted at the focus of the reflector 10 because its aperture is very much smaller than the aperture of the paraboloidal reflector. The dipoles 14 are elements which are generally mounted on the reflector surface with their maximum direction of radiation perpendicular to the proximate reflector surface, that is the surface of the reflector proximate to the dipole elements. If the F/D ratio of the reflector were small so that there would be a large angle between the outer dipole maximum radiation directions and the axis of the paraboloid, the dipoles would then be inclined in a direction more nearly parallel to the reflector axis. This angle would be a function of the beamwidth of the individual dipole 14 and the characteristics of its radiator.

By mounting the elements on a curved surface, it is seen that a certain phase error is introduced as compared with the mounting of such dipole elements on a flat surface, and this is compensated for by phase compensation means 16, 16 introduced in the transmission lines 18, 18 that feed the array of dipoles 14, 14. The electrical lengths of the transmission lines respectively feeding each of the dipoles 14, 14, are constructed and arranged such that the phase distribution across the aperture of the paraboloid 10 is constant. The net result of this arrangement is that a radiation pattern having a high gain and a side lobe level which is accurately calculable results, although a blocking aperture effect of the dipole elements may be a large percentage of the total aperture than a centrally located feed means. The effect of the radiation pattern is less for at least two reasons. One is that the elements are widely dispersed so that they do not result in an effective high gain aperture, and the second reason is that the elements are located mostly in a region of low illumination. A centrally located blocking aperture is in an area whose field concentration is approximately 10 times as great as the edges of the reflector, and the amplitude and phase of excitation of the two-dimensional array is very accurately controllable by the phase compensating means 16, 16 or branching network feeding the individual elements.

The array of two-dimensional dipoles may be connected to form a pencil beam for the reflector 10, or a beam for monopulse tracking systems, or it may be connected through mechanical or electronic phase shifters for very rapidly producing a substantially conical scan or sequential lobing.

All of the applications described above apply not only to a single paraboloidal reflector, but also to other types such as Cassegrainian, Gregorian, or various types of surfaces such as paraboloidal cylinders and paraboloidal toruses, or at least where a portion of the reflector surfaces is so defined.

The array of two-dimensional dipoles forming the elements along the reflector surface of the antenna reflector 10 may be composed of crossed dipoles as mentioned above, or the array may be componed of such other forms of radiating elements such as helices, log periodic, or frequency independent types of elements as well as other types of end-fire arrays such as cigars or yag1s.

Additional embodiments of the invention in this specification will occur to others and therefore it is intended that the scope of the invention be limited only by the appended claims and not by the embodiment described hereinabove. Accordingly, reference should be made to the following claims in determining the full scope of the 55 invention.

What is claimed is:

1. A broadband antenna comprising a reflector defining a surface of a parabola in at least one cross-sectional plane thereof, a microwave feed means positioned near the focus of the reflector to provide a beam directed upon its surface, a series of two-dimensional crossed dipole radiation arrays mounted upon the surface of said reflector, said series mounted to intersect with a given intersecting plane of said reflector, said series of dipole arrays having their maximum direction of radiation generally perpendicular to the surface of said reflector, and phase error compensation means in the transmission lines feeding said series of dipole arrays so that the phase distribution across the aperture of the reflector is generally constant.

2. A broadband antenna comprising a paraboloidal surface reflector, a microwave feed means positioned at the focus of the surface of the reflector for providing a beam directed upon the paraboloidal reflector surface, a series of two-dimensional crossed dipole radiation arrays mounted upon the surface of said reflector, said series of dipole arrays having their maximum direction of radiation generally perpendicular to the proximal surface of said reflector, feed lines for each of the dipoles of said series and of the arrays, and phase error compensation means in each of the feed means feeding said series of dipole arrays so that the phase distribution across the aperture of the reflector is calculable.

3. A broadband antenna comprising a reflector defining a parabola in at least one cross-section of the reflector, a microwave feed means positioned proximate the focus of the reflector to provide a beam directed upon the surface of the reflector, a series of two-dimensional dipole radiation arrays mounted upon the surface of said reflector, said series of dipole arrays having their maximum direction of radiation generally perpendicular to the surface of the reflector from where the respective dipoles are mounted, and phase error compensation means in the transmission lines feeding said series of dipole arrays so that the phase distribution across the aperture of the reflector is generally constant.

References Cited UNITED STATES PATENTS 2,367,764 1/1945 Ferris 343753 2,566,703 9/1951 Iams 343753 3,045,237 7/1962 Marston 343-754 3,245,081 4/ 1966 McFarland 343-754 2,846,678 8/1958 Best 343840 2,895,127 7/1959 Padgett 343--840 ELI LIEBERMAN, Primary Examiner.

U.S. Cl. X.R. 

